Capacitors of various types are well known and useful in a wide variety of electronics applications. There are a wide variety of types of capacitors, each having a particular arrangement of conductors and dielectrics used to secure capacitance for the storage of electrical energy in an electric field. An essential feature of all capacitors is a system of conductors separated by dielectrics.
In this regard, one particular type of capacitor is termed a "wound capacitor". Wound capacitors comprise a pair of wound bilaminar ribbons each of which has a layer of dielectric substance, e.g. polyester or polystyrene, upon which a conductive layer, e.g. metal, is deposited. The pair of ribbons are partially overlapped and wound together so that one edge of each ribbon forms a contact for the capacitor. When completely wound, the capacitor takes on the general shape of a cylinder with either end functioning as a contact.
Generally, wound capacitors are superior to other types of capacitors, e.g. ceramic capacitors, because of their improved performance and self-curing characteristics. For example, wound capacitors may be self-curing, that is a short or defect affecting the performance of a part of the capacitor may not significantly affect the performance or operation of the capacitor as a whole. In spite of the performance advantages of wound capacitors over some other capacitors, e.g. ceramic capacitors, wound capacitors have been limited in their application in some areas because of the difficulty or inability to incorporate them in various circuitry due to connection problems. Accordingly, conventional manufacture and design of wound capacitors have not fully addressed these problems.
In addition to these problems, various defects affect the efficiency and, therefore, the performance of a wound capacitor. For example, moisture trapped between the layers of the capacitor may cause shorting in the capacitor. Likewise, air gaps between its layers may cause ionization in the capacitor reducing the performance of the capacitor. These defects are inherent in the winding process. Generally, the ribbons of the capacitors cannot be wound tight enough by conventional winding means to eliminate these defects without damaging the ribbons or causing other defects. Accordingly, in conventional capacitor manufacturing the capacitors must be processed by heat curing them in a time-consuming manner to remove substantially all of the defects that inherently result from the winding process.
In conventional wound capacitor manufacturing, defects that inherently result from winding the capacitors, e.g. moisture and air, are removed from the capacitor by mechanically compressing the capacitor for a period of time, approximately one hour, and then heat-curing the capacitor at a high temperature, approximately 145.degree. C., for approximately twelve hours or other appropriate extended period of time. Accordingly because of factors, such as the significant time factor involved in the conventional curing process, the cost of manufacture of wound capacitors can be relatively high.
Another factor in wound capacitor manufacturing and design is the method by which the wound capacitor can be connected to electronic circuitry, e.g. circuit boards. It is economical an time efficient to be able to surface mount electric components and utilize various soldering techniques, e.g. wave soldering to connect them. However, wound capacitors are often not designed for surface mounting nor are they necessarily compatible with some soldering techniques, e.g. wave soldering. For example, a wound capacitor of conventional design may not be surface mounted using wave soldering methods because the wave soldering method may cause damage to the wound capacitor unless it is protected. Further, the surface mounting and some soldering methods, e.g. wave soldering, often require relatively precise lead spacing, a feature often not available with conventional wound capacitors. As a result of these problems, many users turn to ceramic capacitors in spite of the inferior performance they offer. Some conventional methods of wound capacitor manufacturing and design have attempted to address these problems.
In this regard, some conventional capacitor manufacturing and design have enabled wound capacitors to be wave soldered to the electronic circuitry. In these methods and designs, usually the ends of the capacitor are metallized by schooping, a process in which the end surfaces of the capacitor are coated with metal by spraying them with the molten metal shot from a nozzle with compressed air. Wire leads are the metallized conductive ends of the capacitor. Thereafter, the capacitor is placed in a protective preformed case, and the case is filled with epoxy to seal the capacitor in the case. The need for a preformed case in conventional manufacturing methods contributes significantly to the cost of the process. Further, utilization of these methods and capacitor designs may result in various defects in the capacitor, e.g. in misaligned wire contacts, bubbles in the epoxy filling, non-adherence of the epoxy to the casing, which may result in diminished performance, unusable or nonfunctional capacitors.
Accordingly, there exists a need for a more rapid and economical process for making wound capacitors. In this regard, the present invention provides a more rapid and economical process for making wound capacitors as well as a design for a wound capacitor that enables the capacitor to be surface mounted on electronic circuitry and soldered by various methods, including wave soldering.